Summary  
Common Table Expressions (CTEs) let you define temporary, named result sets within a single SQL query to break complex logic into clear, reusable building blocks and improve readability and maintainability.  

General domain of usage  
Database querying

Common Table Expressions, or **CTEs**, are a powerful feature in SQL that allow you to define a temporary, named result set within the execution scope of a single query. You use CTEs to organize complex queries, break them into logical building blocks, and improve both readability and maintainability. **CTEs** are especially useful when you need to reference a derived table multiple times in a query, or when you want to simplify deeply nested subqueries. Whenever you find yourself struggling to follow a long query with multiple subqueries, or you need to reuse a calculated result, a **CTE** can make your SQL much clearer and easier to manage.

WITH HighEarners AS (
    SELECT employee_id, first_name, last_name, salary
    FROM employees
    WHERE salary > 55000
)
SELECT *
FROM HighEarners;

Let's break down the syntax of a CTE step by step. You start with the `WITH` clause, which signals the beginning of one or more CTE definitions. Next, you provide a **CTE name**, such as `HighEarners`, followed by the CTE's query enclosed in parentheses. This query defines the temporary result set. After the CTE is defined, you write your main query, which can reference the CTE just like a regular table. In the example above, the main query simply selects all columns from `HighEarners`. By naming the CTE, you make your query's intent clear and allow for easy reuse of the derived result.

-- Using a subquery
SELECT employee_id, first_name, last_name
FROM (
    SELECT employee_id, first_name, last_name, salary
    FROM employees
    WHERE department_id = 1
) AS SalesEmployees
WHERE salary > 55000;

-- Using a CTE for clarity
WITH SalesEmployees AS (
    SELECT employee_id, first_name, last_name, salary
    FROM employees
    WHERE department_id = 1
)
SELECT employee_id, first_name, last_name
FROM SalesEmployees
WHERE salary > 55000;

What is the main advantage of using a CTE over a subquery?

Which SQL keyword is used to define a CTE?

Can a CTE be referenced multiple times within the same query?

A comprehensive course designed to help you master Common Table Expressions (CTEs) in SQL, from foundational concepts to advanced real-world applications. Explore recursive queries, modular query design, and performance considerations through engaging theory and practical, scenario-based tasks.

Explore the basics of Common Table Expressions, their syntax, and how they can simplify SQL queries. Practice using CTEs in real-world scenarios such as reporting and data transformation.

Delve into recursive CTEs for handling hierarchical data, such as organizational charts and bill of materials. Practice building recursive queries to solve real-world problems.

Master advanced CTE techniques, including combining CTEs with aggregates, optimizing performance, and using CTEs for complex reporting.

Introduction to CTEs: Syntax and Benefits

1. What is the main advantage of using a CTE over a subquery?

2. Which SQL keyword is used to define a CTE?

3. Can a CTE be referenced multiple times within the same query?